In a game, your players tend to expect their plans to fall apart quickly, for everything to hit the fan, including the brown stuff. That’s why, when things go smoothly, like the plan in “Target of Opportunity,” the players get nervous—just like Scott and Becca. Once in a while, it might be fun to mess with the players’ heads by letting things work out, but there should be a reason why things go well, not just GM fiat.
The reverse holds true—don’t throw spanners into your players’ works just because you can. Everything in the game should follow logically.
If your players have crafted an excellent plan, one that takes into account a variety of factors, the only reason the GM should have to scotch it, is if that is based on the logic of the campaign. For example, maybe the terrorist the characters intend to snatch always has three snipers in overwatch when he’s addressing his followers. The characters may have planned to deal with two of the snipers, but have missed the third. Or they missed the second set of laser sensors inside the safe. Or the badguy doesn’t like cherry sherbert.
All of these are part of the campaign, part of the GM’s plan. It’s unfortunate that they might ruin the characters’ plan, but that’s just how it works. This also means that it’s important that the GM understand the NPCs. For example, if there’s no particular reason why the badguy wouldn’t like cherry sherbert, deciding it is so in order to ruin an otherwise excellent plan by the characters is cheap. If there’s drugs in the pork, and the NPC is Jewish or Muslim, that’s something else.
There’s always luck, or the lack of it. Unfortunately, that might seem too much like—or might actually be—GM fiat. This might be okay once or twice in a campaign, but the success or lack there of shouldn’t be predicated on the GM’s feelings or desires.
If you, as the GM, feels that the players have gone above and beyond in their preparations, or perhaps in their gaming, you might want to cut them a break and call it luck. That’s fine. I don’t think the reverse is true. I don’t think that the GM should punish players by decreeing bad luck for their characters. If the characters are unprepared, there shouldn’t be the need to invoke bad luck to watch their plans crumble.
Invoking bad luck will seem too much like sour grapes to the players. Remember, this is not a competitive game, it is a collaborative game. By using GM fiat to insure the players’ plans fail seems like cheating to win. Whether that is true or not, the perception could arise. That’s going to change the complexion of the game. When a GM loses the trust of the players, the fun suffers.
In the end, you want to challenge the players, not crush them. You want them to be able to be successful, but it should never be easy. When things work out, even when it is due to excellent preparations on the players part, it might actually make them sweat. No one should expect their plan to survive contact with the enemy.
And, yes, once in a while, just to play with their heads, it can be as fun to allow the players’ plan to succeed spectacularly.